How can I tech my 11 year old daughter not to expose herself???

Hi, my daughter who is now 11 and has just started at a special secondary school, has no embarrassment what so ever when it comes to nudity. She has moderate learning difficulties and possibly a form of autism. Things like, if her trousers fall down, I have to nag her to pull them up, or she won't. She doesn't care that she's showing her backside in a public place. It is worrying! She was recently sitting crossed legged in the living room, in her nightdress, with no knickers and had everything on show to anyone who walked passed the window. When I told her off, she just said she doesn't care who see's her and that she wont be able to hear, through the closed window if anyone makes fun of her, so it doesn't matter. The other day, while talking to her uncle, she pulled up her leggings, by pulling them down first to grab hold of the material, but showing the front of her knickers as she did it. I told her off, but she did the same thing again that day, so we did punish her by not staying at his house to watch her dance. She likes to put on dance productions for us. This upset her, but she still doesn't know why she shouldn't expose herself, saying about her uncle "But I know him! He's my uncle!" I've tried showing her the NSPCC advert which deals with this, but she just found it funny. She is a very tall girl for her age and is already very well developed, and even that doesn't bother her when it comes to exposing herself. I'm not sure the school are aware of this or weather or not to tell them? Maybe they could help? I'd be really grateful for any advice, as I'm not sure how to tackle this one. It is a worry!Thanks.

I've googled it and all the answers seem to point to the exposing being some kind of sexual thing. This is certainly not the case with my daughter. It's literally a 'couldn't care less' attitude to nudity. She wouldn't flash at someone on purpose, and sees nothing funny about nudity, to her, showing her privates would be the same as showing, say her elbow...